Lubricating oils, such as motor oils, gear oils, hydraulic fluids and transmission fluids typically contain several additives to improve their performance. These can include dispersants, antioxidants, detergents, friction modifiers, de-foaming agents, viscosity index improvers, and pour point depressants.
Lubricants, especially those oils obtained from petroleum via distillation, generally contain long-chain n-alkanes, which are known to precipitate out in crystalline form upon cooling. The presence of these waxy paraffins significantly reduces the fluidity of the oils at low temperature conditions. The crystallization or precipitation of the paraffin components can have an adverse effect on the flow of the lubricating oil or even completely prevent flow. An improvement in the pour point (the lowest temperature at which point the formulated oil remains fluid) of a lubricant can be achieved by partial dewaxing, and then this pour point can be further lowered by the addition of polymeric flow improvers called pour point depressants or pour point improvers. These pour point depressants are effective at low concentrations, for example in the 0.05 to 1 weight percent range in the oil.
The exact mechanism by which these polymers function is not straightforward but it is presumed the paraffin-like polymers become incorporated into the growing paraffin crystal structure and disrupt further crystal growth. This disruption prevents the formation of extended crystal agglomerates thus allowing the oil to remain fluid at lower temperatures than possible in the absence of these compounds. Certain facets of a polymers structure are known to impart pour point depressant activity. For example, polymers with long alkyl side chains are known to exhibit a pour point and flow improving effect as the alkyl groups are able to disrupt crystal growth (see Ullman's Encyclopedia of Industrial Chemistry, 4th Edition, Volume 20, Verlag Chemie, 1981, p548). Other desired properties in commercial pour point depressant polymers include, good thermal, oxidative, and chemical stability, shear strength, etc.
Obtaining suitable low temperature performance has become even more difficult recently with the movement away from API Group I base oils and the increased utilization of Group II and Group III base oils. Further, refiners who blend with different base oils desire a single product, which performs effectively in all of these different base oils. The present invention is directed to novel, controlled acrylic copolymers, which exhibit excellent low temperature performance in a wide variety of base oils.
Random copolymers are used to a wide extent as pour point depressants. One prevalent class of materials is polymethacrylates (PMA's) that contain long chain alkyl residues. These compounds are described, for example, in U.S. Pat. Nos. 2,091,627, 2,100,993, 2,114,233 and EP-A-0 236 844. In general, these pour point depressants are produced by radical polymerization. To ensure solubility in a lubricating oil base, these copolymers typically contain high levels of a monomer or monomers whose homopolymer is oil-soluble.
Polymers having a controlled architecture, including star copolymers (EP 1064347) and block copolymers have been described in the art. These polymers can be prepared through a variety of living anionic and living (or controlled) free radical polymerization techniques. These techniques have been used primarily to control the molecular weight distribution.
U.S. Pat. No. 6,538,091 describes a process for controlling a polymer architecture using an atom transfer process (ATRP) based on a redox reaction with a transition metal compound. This process uses an initiating system resulting in a copolymer having a predictable molecular weight and allows for some control over polydispersity. Polymers made by the process are described as useful for molding materials, barrier materials, thermoplastic elastomers, and amphiphilic surfactants. This controlled radical polymerization technique has several drawbacks such as, residual metallic by-products which can be detrimental to many applications (for example see U.S. Pat. No. 6,610,802) and limitations in polymer composition.
Random copolymers made by ATRP have been used as pour point depressants (U.S. Pat. No. 6,391,996), and viscosity index improvers (US2002/0188081). The '996 patent mentions that the ATRP process could be used for blocky copolymers, but it fails to exemplify such a use, or disclose the PPD benefit of using such block copolymers in lubricating oils. Also, U.S. Pat. No. 6,403,745 discloses gradient copolymers synthesized by ATRP useful as pour point depressants. U.S. Pat. No. 6,403,745 discloses gradient copolymers, not block copolymers. Gradient polymer structures are random copolymers that rely on monomer feed rates to obtain a gradient structure with the controlled nature of the polymerization providing homogeneity of composition across all the polymer chains (polymers formed by traditional methods contain a statistical mixture of copolymers).
The controlled polymers disclosed in U.S. Pat. Nos. 6,391,996 6,403,745 refer to the use of initiators having a transferable atomic group and a catalyst containing a transition metal (ATRP techniques). These types of polymerizations have several drawbacks including, but not limited to, slow polymerization kinetics, residual metallic byproducts, and limited polymer composition and molecular weight ranges. The metallic by-products are detrimental in engine-type lubricant applications and require removal, which is difficult and requires laborious procedures. U.S. Pat. No. 6,610,802 describes these byproducts and discloses the disadvantage of ATRP processes. The residual metal catalysts can be detrimental for many applications as they influence the product properties and impact environmental compatibility.
Block copolymers have also been shown to be useful as viscosity index improvers (VIIs). Block copolymers of a vinyl aromatic monomer and a vinyl aromatic-co-acrylic block prepared by stabilized free radical polymerization are described in patents EP 0 945 474, and U.S. Pat. No. 6,531,547. These patents describe the use of TEMPO-based nitroxide derivatives for the synthesis of the corresponding block copolymers. It is well known that this class of free radical control agent does not provide control over acrylic type monomers. Specifically, the use of methacrylics will lead to side and termination reactions such as disproportionation, which inhibits the formation of block copolymers and long chain molecules (as described by Ananchenko et. al. in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40 pp 3264-3283). Also, block copolymers of ethylene and alpha-olefins have been described in US 2003/0073785 and block copolymers of poly(conjugated dienes) and poly(monovinyl aromatic hydrocarbons have been described in U.S. Pat. No. 6,303,550. These references do not disclose the use of a controlled architecture copolymer having at least one pure acrylic block segment for use as a VII.
U.S. Pat. No. 5,002,676 describes the preparation of block copolymers containing selectively hydrogenated conjugated dienes and t-butyl methacrylate. U.S. Pat. No. 6,350,723 teaches the synthesis of block copolymers through the living anionic polymerization of a conjugated diene and an alkyl methacylate monomer. These references exemplify the use of block copolymers containing conjugated dienes and hydrogenated dienes. Also these reports do not teach the significance of tailoring block composition or allow for the formation of gradient compositions. Furthermore, living anionic polymerization suffers from several drawbacks, such as, ineffectiveness at temperatures above −20° C., poor copolymerization between polar and non-polar comonomers, and the inability to use monomers that can be easily deprotanated. Therefore functional monomers cannot be incorporated, and the copolymerization of monomer mixtures can be problematic and/or unusable. Furthermore this process can be expensive and difficult or impractical to carry out on an industrial scale as bulk or emulsion techniques cannot be used, extremely pure reagents are necessary (even trace amounts of protic material inhibits polymerization), and an inert atmosphere is requisite.
A process for preparing copolymers in the presence of a stable free radical from the nitroxide family is described in U.S. Pat. No. 6,255,402. Nitroxide-mediated stable radicals have been used to produce controlled block copolymers, as described in U.S. Pat. No. 6,255,448, US 2002/0040117, and US Published Application No. 2005/0107577. These references, incorporated herein by reference, do not disclose the use of the copolymers in lubricating oils.